1. Field
This disclosure is generally related to a passive optical network (PON). More specifically, this disclosure is related to performance monitoring in a PON.
2. Related Art
In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity is also needed to meet the increasing bandwidth demand of end users for triple play services, including Internet protocol (IP) video, high-speed data, and packet voice. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering large bandwidth to end users.
Among different competing technologies, passive optical networks (PONs) are one of the best candidates for next-generation access networks. With the large bandwidth of optical fibers, PONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, PONs can be built with existing protocols, such as Ethernet and ATM, which facilitate interoperability between PONs and other network equipment.
Typically, PONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and the premises of the customers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of customers. For example, a PON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to customers and combines upstream optical signals from customers (see FIG. 1). Note that other topologies are also possible including ring and mesh topologies.
Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. The ONU can reside in the residence of the customer and couples to the customer's own home network through a customer-premises equipment (CPE). Sometimes, an ONU is also referred as an optical network terminal (ONT), which terminates the PON and presents the customer-service interface to users. In this disclosure, the term “ONU” refers to ONU, ONT, or any other downstream node equipments in a PON.
In the example of an Ethernet PON (EPON), communications can include downstream traffic and upstream traffic. In the following description, “downstream” refers to the direction from an OLT to one or more ONUs, and “upstream” refers to the direction from an ONU to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data packets are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. Moreover, each ONU is assigned one or more logical link identifiers (LLIDs), and a data packet transmitted by the OLT typically specifies an LLID of the destination ONU. If the data packet is a broadcast packet destined to all ONUs, then it will specify a broadcast LLID. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT.
In order to avoid collision of upstream transmissions from different ONUs, ONU transmissions are arbitrated. This arbitration can be achieved by allocating a transmission window (also called a grant) to each ONU. An ONU defers transmission until its grant arrives. A multipoint control protocol (MPCP), which resides in the media access control (MAC) control layer, can be used to assign transmission time slots to ONUs. MPCP employs REPORT (an upstream message from the ONU to inform its queue information to the OLT) and GATE (a downstream message from the OLT to grant bandwidth to ONUs) control messages to request and assign transmission opportunities on the EPON.
FIG. 1 illustrates a passive optical network including a central office and a number of customers coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter 102 and optical fibers couple the customers to a central office 101. Multiple splitters can also be cascaded to provide the desired split ratio and a greater geographical coverage. Passive optical splitter 102 can reside near end-user locations to minimize the initial fiber deployment costs. Central office 101 can couple to an external network 103, such as a metropolitan area network operated by an Internet service provider (ISP).
Although FIG. 1 illustrates a tree topology, a PON can also be based on other topologies, such as a logical ring or a logical bus. Note that, although in this disclosure many examples are based on EPONs, embodiments of the present invention are not limited to EPONs and can be applied to a variety of PONs, such as ATM PONs (APONs), gigabit PONs (GPONs, which are PONs using a variant of a generic framing protocol), and wavelength division multiplexing (WDM) PONs.
In order to provide low-cost, high-bandwidth, and reliable service to customers, a PON needs to remain reliable and cost-efficient approach. PON maintenance should ideally provide proactive and continuous monitoring of network health without service disruption, and perform fault diagnosis of common failures in optical transceiver modules, as well as in the optical distribution network (ODN) fiber segments and passive splitter elements.